In recent years, developments have been made in processes for compacting polymeric strands in order to make sheets of high stiffness and strength. An example is disclosed in GB 2253420A, in which an assembly of strands of an oriented polymer is hot compacted in a two-step process to form a sheet having good mechanical properties. The process involves an initial step in which the strands are brought to and held at the compaction temperature whilst subject to a pressure sufficient to maintain the strands in contact, and thereafter compacted at a high pressure (40-50 MPa) for a few seconds (the compaction pressure). In this process a proportion of the surfaces of the strands melts and subsequently recrystallises on cooling. This recrystallised phase binds the strands together, resulting in good mechanical properties of the final sheet. It is mentioned in GB 2253420A that the process can be applied to many types of oriented polymer including polyester and PEEK (polyether ether ketone) but that preferred polymers are oriented polyolefins.
One drawback of the process described in GB 2253420A is that the temperature span across which melting occurs is very narrow. Accordingly it is difficult to achieve the desired degree of partial melting of the outer regions of the strands. Inadequate melting of the strands results in poor mechanical properties. Excessive melting of the strands results in loss of orientation, and diminished mechanical properties. Precise process control is needed if the article is not to be “under-melted” or “over-melted”.
In WO 98/15397 a related process is disclosed in which an assembly of melt-formed polyolefin strands is maintained in intimate contact at elevated temperature sufficient to melt a proportion of the strands, whilst being subjected to a compaction pressure of no greater than 10 MPa. If wished the strands may have been subjected to a prior crosslinking process, preferably an irradiation crosslinking process comprising irradiating the strands with an ionising radiation in an inert environment containing alkyne or diene compounds, and then carrying out an annealing step comprising annealing the irradiated polymer at an elevated temperature, in an inert environment containing alkyne or diene compounds. It is said that the prior crosslinking can make the compaction temperature less critical, and improve mechanical properties, in particular the failure strength at elevated temperature.
There is published work on the use of articles in which a polyethylene film is sandwiched between polyethylene fibre layers, and the ply subjected to hot compaction.
Marais et al., in Composites Science and Technology, 45, 1992, pp. 247-255, disclose a process in which compaction takes place at a temperature above the melting point of the film but below the melting point of the fibre layers. The resulting articles have modest mechanical properties.
Ogawa et al., in Journal of Applied Polymer Science, 68, 1998, pp. 1431-1439 describe articles made up of layers of ultra-high molecular weight polyethylene fibres (mp 145-152° C.) and low density polyethylene films (mp 118° C.). The moulding temperature is said to be between the melting points of the fibre and the interlayer (matrix). The volume fraction of the fibres is stated to be 0.69 or 0.74. However the articles are said to have surprisingly poor properties, possibly because of weak adhesion between fibres and matrix (melted film) Another article was made with polyethylene fibres alone, and the process conditions induced partial melting, with poorer properties.